1. Field of the Invention
The present invention-generally relates to systems for securing portions of a patient's spinal column into a desired fixed position to correct injuries and defects and, more particularly, is concerned with a spinal fixation system which is adjustable when implanted and is resistant coming apart or detached from the patient's spine after implantation due to the patient's movement.
2. Description of the Related Art
Spinal fixation systems have been used for some time to secure sections of the spinal column, individual vertebral bodies and the like, into a fixed position to correct spinal injuries and defects. For example, spinal fixation systems have been used to correct injuries to the spine where an individual vertebral body has been shifted either laterally or vertically from its desired position.
Typically, in these applications, spinal fixation systems include a screw or hook which is attached to a portion of the vertebral body. At least one screw is attached to a first vertebral body which is in a correct position and is positioned on one side of the incorrectly positioned vertebral body. At least one screw or hook is then attached to a second correctly positioned vertebral body which is positioned on the other side of the incorrectly positioned vertebral body. Rods are then shaped so that they interconnect the hooks or screws in the first and second correctly positioned vertebral bodies. The surgeon then moves the incorrectly positioned vertebral body into a desired position, inserts a hook or screw and connects the hook or screw to the support rod so that the incorrectly positioned vertebral body is retained in its desired position.
One difficulty that spinal fixation systems must be able to overcome is the tremendous amount of forces that are exerted on the system once the system is implanted into a patient. As can be appreciated, the continuous movement of the patient's back during the average day exerts enormous forces against the spinal fixation system. This requires the spinal fixation system to be both securely fastened together and securely attached to the vertebral bodies.
A further difficulty associated with spinal fixation systems is that major surgery is required to install the system. This surgery includes the dissection of the patient's back until the affected vertebral bodies are exposed and then installation of the spinal fixation system. It is desirable to minimize the amount of time that the vertebral bodies are exposed. Unfortunately, however, installation of most prior art spinal fixation systems is time consuming which increases the risks associated with major surgery of this type.
Specifically, to implant the spinal fixation system, the screws or hooks must be attached to the various vertebral bodies. Subsequently, these screws or hooks must be connected to the support rods. However, the support rods typically have to be bent into required shape to be connect to the screws and hooks. Since the screws and hooks cannot be moved once they are attached to the vertebral bodies, the rods generally have to be bent to a specific shape before they are attached to the screws and hooks. This bending is normally done during surgery, while the vertebral bodies are accessible to the surgeon. This is due to the fact that the surgeon must adjust the bend of the rods in a trial and error manner until the system aligns the bodies as desired. Consequently, much time is expended during the course of the surgery shaping the rods to the exact tolerances needed to attach the screws and hooks to the rod.
To minimize the amount of time expended in shaping the rods, efforts have previously been made to develop an adjustable spinal fixation system. For example, U.S. Pat. No. 5,261,909 to Sutterlin et al. discloses a variable angle screw for a spinal implant system which reduces the amount of shaping that must be done with the rods when implanting a spinal fixation system. Sutterlin et al., discloses a bone screw which is yolked on the top to permit top loading and which has a plurality of radially extending splines. The bone screw is positioned in the vertebral body in a well known fashion and it is attached to the rod in the following manner.
The rod is inserted into an eyebolt assembly and the rod and eyebolt assembly is positioned adjacent the bone screw. A washer having two grooves on one side is then positioned around a shaft of the eyebolt, adjacent the rod, so that the rod rests in the grooves. The side of the washer opposite the rod has a plurality of radially extending splines which mate with the splines on the bone screw when the eyebolt and rod assembly is positioned adjacent the bone screw. A nut is then positioned on the shaft of the eyebolt and, when tightened, the nut urges the bone screw splines into the washer splines and also urges the washer to clamp the rod against the eyebolt.
The radially extending splines allow the surgeon to rotate the eyebolt with respect to the rod during attachment of the fixation system to the vertebral bodies. Thus, the surgeon may interconnect the bone screw and the support rods without having to shape the rod so that the eyebolt is exactly perpendicular to the bone screw at the point of attachment.
However, the bone screw assembly disclosed in the Sutterlin, et al. reference still requires the surgeon to shape the rod to ensure that the support rod is vertically aligned with the yoke of the bone screw. Specifically, the support rod must be shaped so that, at the point of attachment, the rod is the same distance from the vertebral body as the splined portion of the bone screw to permit the radially splined washer to align and interconnect with the radially splined portion of the eyebolt. Hence, the surgeon still has to spend valuable time shaping the support rods during the surgical procedure so that the fixation system can be properly implanted.
A further problem with the Sutterlin device is that, when the bone screw is attached to the rod, the rod is held in position only by virtue of the grooves clamping the rod to the interior surface of the eyebolt. If enough force is exerted on the rod, the rod can be induced to rotate in the grooves. Motion of the spine during normal human activities, e.g., walking, turning from side to side etc., can result in tremendous forces being exerted on an implanted spinal fixation system. These forces can be strong enough to induce the support rod to rotate or twist which may result in misalignment of other components, pressure on the spinal column, or damage to the components of the implanted spinal fixation system.
Still another problem with the Sutterlin spinal fixation device is that during installation, the surgeon has to be able to access the nut from the side to tighten and connect the bone screw to the rod. This requires the surgeon to make a larger incision to allow side access to the nut by a wrench. Further, the surgeon can have difficulty in tightening the nut using a wrench in the confined spaces adjacent the vertebral body, and this can result in a longer operation with increased risk to the patient.
For these reasons there is a need in the art for an improved spinal fixation system which is configured so that bone mounting devices, such as hooks and screws, which are attached to the vertebral bodies can be securely connected to a rod assembly with no or minimal bending of the rod and without requiring exact rotational or vertical alignment between the bone mounting devices and the rod assembly. Further, there is a need for an improved spinal fixation system which can be configured to prevent rotation or movement of the implanted rods due to movement of the patient. Finally, there is also a need in the art for an improved spinal fixation system which can be both directly implanted through a minimal incision, and also tightened without increasing incision size for such tightening to further minimize the amount of time, effort and trauma needed to implant the system.